ELECTROPHYSIOLOGICAL AND FOS IMMUNOHISTOCHEMICAL EVIDENCE FOR THE EXCITATORY NATURE OF THE PARAFASCICULAR PROJECTION TO THE GLOBUS-PALLIDUS

Extracellular recordings and immunohistological detection of c-Fos-lik e immunoreactive proteins were used to determine the synaptic effect o f the parafascicular projection to the globus pallidus. Electrical sti mulation of the parafascicular neurons induced a single-spike excitato ry response with a stable latency of 2.3 ms, suggesting a monosynaptic ally driven effect. Pharmacological stimulation of the parafascicular nucleus with carbachol increased tonically the pallidal discharge rate by 142%. The discharge rate of the pallidal neurons was decreased by 37% in parafascicular-lesioned rats. These results demonstrate the exc itatory nature and the tonic action of the parafasciculopallidal proje ction. Carbachol activation of parafascicular neurons also induced the synthesis of c-Fos-like immunoreactive proteins in the pallidal neuro ns. Control experiments in subthalamic-lesioned rats showed that the p arafascicular excitation of the pallidal neurons remained, but both el ectrophysiological and expression of c-Fos-like immunoreactive protein s were attenuated. This suggests that the direct parafascicular excita tion of the pallidal neurons is indirectly reinforced by the previousl y described parafascicular excitatory input to the subthalamic nucleus . Conversely, the effect of this last input to the subthalamic nucleus is dramatically enhanced in rats with pallidal lesion. Our results de monstrate the complex role of the parafascicular nucleus in activating both the globus pallidus and the subthalamic nucleus, two closely rel ated structures. These results illustrate the integrative capacities o f the globus pallidus, whose activity is modulated by multiple afferen ts. (C) 1997 IBRO. Published by Elsevier Science Ltd.